This invention relates to electronic pulse width monitors and, more particularly, to such monitors which provide an output signal indicative of whether successive voltage pulses occurring over a predetermined time period have a duration which is less than a preselected duration.
AC electric power systems are usually connected in parallel to increase total system rating or in certain cases such as airborne power systems, to increase reliability. One well-known type of aircraft electric power system is the DC link variable speed constant frequency (VSCF) system. Such systems include a plurality of power pole switching elements which synthesize an AC output by switching current from a pair of DC link conductors in a fixed switching pattern, which may be generated by a microprocessor or other digital circuit When these systems are to be operated in parallel with each other or with another source such as a ground power cart, each of the power sources must be synchronized. Since DC link VSCF systems are clock-based, a master clock signal is used to synchronize each of the system channels
The clock provides a single digital signal used to synchronize each channel. No break power transfer (NBPT) is a special application of a parallel system which allows momentary paralleling with another power source. The other source is typically an auxiliary power unit (APU) or a ground power unit (GPU). For NBPT, the master clock should be able to track the frequency of the external source. The master clock system includes a phased locked loop with selectable inputs from the APU, GPU, or a crystal frequency reference counted down to 400 Hz.
During NBPT, there is a transient due to mismatched phase angles and magnitudes of the two system voltages about to be momentarily paralleled. Severe transients and tripping of the VSCF system could result if excessive phase error exists prior to NBPT. To minimize such transients, it is necessary to ensure proper phase angle matching prior to paralleling. A phase detector is used to provide an error signal that is representative of the phase error between the VSCF system and the selected reference input.
One type of phase detector, as illustrated in U.S. Pat. No. 4,849,704 issued July 18, 1989, provides a phase error signal in the form of a pulse train, with the widths of the individual pulses being proportional to the phase error. A circuit is needed to monitor the widths of the pulses in the phase error signal and to provide an indication if the phase error exceeds a preset value to prevent NBPT.